Most people can update their desktop browser for free at any time. Most (85%) of the Android users out there are stuck on something <= 2.3.
And it depends on your audience. For instance, if you're Reddit (or are targeting the Reddit users) you're looking at 7% on IE, and 2% on Opera. You can practically ignore those browsers in that case.
Most people could update their browser for free but that doesn't mean that they will update it. I regularly encounter Firefox 3.x users. Fortunately, Firefox 3.x is still a decent browser in terms of standards support but it certainly isn't anywhere near the latest available version.
I bet the Reddit crowd are almost all up-to-date with whatever browser they are running. A stodgy enterprise or gov't office might mandate a certain outdated version for internal apps. I'd bet that a small slice of the vast number of websites out there actually target the average user. The ones that do are likely generic portal ("start page") sites, stores with broad appeal (Amazon, Target, B&N), or public services that everybody would access.
And it depends on your audience. For instance, if you're Reddit (or are targeting the Reddit users) you're looking at 7% on IE, and 2% on Opera. You can practically ignore those browsers in that case.